(1) Field of the Invention
The present invention relates to a substrate for magnetic recording medium as well as to a process for producing said substrate. More particularly, the present invention relates to a substrate for magnetic recording medium useful for recording apparatuses of, for example, computer, data processor and the like, as well as to a process for producing said substrate.
(2) Prior Art
As well known, aluminum (Al) alloys (which are non-magnetic material) have been in wide use as a material for a substrate for magnetic recording medium such as a magnetic disc, and magnetic discs using such an Al alloy substrate are popular. An example of such discs is one obtained by forming, on an aluminum-magnesium (Al--Mg) alloy substrate, a nickel-phosphorus (Ni--P) layer by electroless plating, and forming thereon a chromium (Cr) layer, a cobalt (Co) alloy layer and a carbon (C) protective layer in this order by sputtering.
In recent years, there have become popular magnetic recording media having a higher recording density, obtained by forming, on a glass substrate (having a very smooth surface), a Cr layer, a Co alloy layer and a C protective layer in this order by sputtering. Recently, there have also appeared magnetic recording media obtained by forming a thin magnetic layer in the same manner as above, on a vitreous carbon made from a phenol-formaldehyde resin.
Thus in recent magnetic recording and regeneration apparatuses, there are required a large recording capacity and, therefore, a small size and a light weight. Conventional sabstrate for magnetic recording media, however, are unable to satisfy such market needs, and development of a new substarte for magnetic recording medium has been desired.
The above-mentioned Al--Mg alloy substrate has a large specific gravity (about 2.6) and moreover is difficult to reduce its thickness to 1 mm or less; therefore, the substrate and the recording apparatus using the substrate cannot be made small and lightweight. The above-mentioned glass substrate can be made so as to have a thickness of 1 mm or less, but has a large specific gravity of about 2.5 and is not suitable for small size and light weight.
In Japanese Patent Application Kokai (Laid-Open) No. 52116/1991, there is described a process for obtaining a magnetic recording medium of relatively light weight (specific gravity=1.85) by subjecting a phenol-formaldehyde resin to press molding, firing and carbonizing the molded material to obtain a vitreous carbon substrate, and refiring the substrate at 2,000.degree. C. or higher at 1,000 atm or higher to reduce pores. This process, however, uses complicated steps and requires a high temperature (2,000.degree. C. or higher) and a high pressure (1,000 atm or higher), resulting in a high cost; moreover, the substrate produced from a phenol-formaldehyde resin has pores (surface defects) remaining on the surface, although the pores are reduced.
Especially, in case of a thin film disc comprising a substrate and a magnetic layer formed thereon and having a thickness of 0.1 .mu.m or less, pores such as pin-holes on the surface of the substrate (surface defects) become a cause of bit errors during magnetic recording, since a condition of the surface of the substrate is reflected to a surface of the magnetic layer formed thereon.
In Japanese Patent Publication No. 11005/1991, there is described a process for obtaining a disc substrate made of a polyacrylonitrile-based vitreous carbon, a pitch-based vitreous carbon, a lignin-based vitreous carbon, a phenol resin-based vitreous carbon, a furan resin-based vitreous carbon, an alkyd resin-based vitreous carbon, an unsaturated polyester-based vitreous carbon or a xylene resin-based vitreous carbon. This substrate also is lightweight (sp. gr.=1.5) but has surface defects comprising 5 to more than 10 pores of 2 .mu.m or larger, on the surface per disc-like substrate of 130 mm in diameter, and is not suitable for a substrate for magnetic disc.